1 inch vs 1.5 inch Parking Lot Paving Overlay

[imnoengineer]

I am overlaying a large parking lot and have gotten bids from several contractors. All are using SF 9.5 asphalt, but one has suggested 1 inch instead of 1.5 inch. What are the pitfalls of this?

Thanks.

 

[Ron]

With a 1-inch thickness, you will have areas of less that 1-inch and areas of more than 1-inch, as with any thickness. With the 1-inch thickness, you have NO undertolerance that will allow adequate binder to keep the coarse aggregate in place. The result will be raveling and deterioration of the pavement.

It is preferable to have a minimum thickness of 2 to 3 times the largest coarse aggregate size (in your case 9.5mm), but in no case less than 1-1/4" thick.

You don't state where you are located. That makes a difference. If you live in an area where freezing occurs, the 1-inch thickness will not be very durable for freeze-thaw resistance.

While a 1-inch thick overlay can be done, adding that extra 1/4" thickness will go a long way to getting you a better, longer lasting pavement surface.

I would suggest that you have inspection and testing done as well during the construction, making sure they place the asphalt at the right temperature and that it is compacted properly and timely. Also make sure the existing pavement surface is properly prepared to receive the new asphalt. Milling is prefered. In addition, a tack coat should be applied to bond the new asphalt to the old asphalt.

 

[ACtrafficengr]

Thin lifts like this are being used in highway pavement management, but I'm not familiar with their use in parking lots.

The key is, they are used to keep good pavement in good condition. They have no structural strength - they are primarily a wearing and sealing surface. If you already have more tham minimal cracks, they won't work.

The mix design is different than standard asphalt. Also, polymer asphalts are used, not standard binders. Quality control is vital to getting a good product.

Thin lifts are an area where you do it right or don't do it at all. Ask the contractor what experience they have with thin lifts, and if you could visit some that are a few years old and see how they hold up.

Here is an article on the subject:

http://www.betterroads.com/road-science-tutorial-4/

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928